A STUDY IN CAKCINOLOGV. 43 
Correlated with these modifications is the extent to whicli the ejjimera oli 
the post-oral segments are involved in the hranchiostegal fold. In most 
of the Brachyura that I have examined they are more intimately involved 
than is the case either in the Raninidas or in the Uromiacea, and the corre- 
sponding sterna and arthrophragms are modified and exhibit features 
departing more widely from the Astacuran type. A preliminary survey 
leads me to believe that a study of the structure and relations of the first six 
post-oral segments in the Brachyura will yield results of considerable classi- 
ficatory value, but any attempt to discuss the subject would involve a long- 
digression, and it cannot be pursued any farther in this place. 
But, before instituting a com[)arison between the pre-oral sternites and the 
adjacent parts of the exoskeleton in the Dromiacea and Raninidas, it will be 
necessary to make a considerable digression with the purpose of clearing np 
doubtful jioints in the morphology of this region in the Uecapoda reptantia, 
and defining more precisely certain terms which, to say the best of it, are 
used in an ambiguous sense by carcinologists. 
In the Astacura and in Brachyura in general there is in front of the mouth 
a plate, usually of considerable breadth laterally, but of variable extent antero- 
posteriorly, to which is given the name of " epistome." Behind this is the 
triangular, or in Brachyura the more often quadrilateral area known as the 
" mouth-frame." The confusion arising from the loose application of these 
names is really remarkable. They are due to Milne Edwards and Audouin, 
and it will be well to quote the former author in extenso (25, vol. i. p. 251): 
"En arriere des fossettes antennaires, on. voit uno surface plane, plus ou 
moins etendue, qui represente le troisieme anneau cej)halique et qui poste le 
nom d'epistome. L'espace occupe par I'epistome, les fossettes antennaires et 
la base des antennes externes coustitue ce que nous appelons la region 
antennaire Les parties laterales et inferieures de la carapace, que 
nous appellerons regions pterygostomiennes, sont tou jours dirigees plus ou 
moins obliquement en dehors et en haut, et sur la ligne mddiane elles laissent 
entre elles un espace vide qui est occupe par I'appareil masticateur et que 
nous designerons sous le nom de cadre buccal ; tantot ce cadre buccal h la 
forme d'un quadrilatere assez regulier, tantot il est triangulaire, et c'est 
toujours a sa partie anterieure que viennent se terminer les conduits efferens 
des cavites branchiales." The definition of the " epistome " is sufficiently 
exact ; it represents the sternum of the third cephalic segment, that is, the 
antennary segment. Huxley (38) calls the epistome "the sternal region 
which appertains, in part, to the antennae and, in part, to the mandibles." 
He was correct in recognising the inclusion of the mandibular sternal 
element in the "epistome," so we must take the term to connote the con- 
joined antennary and mandibular sterna. Both Milne Edwards and Huxley 
further recognised the distinctness of the antennulary sternum lying in front 
